The skill combinations are battle moves. I've only been playing for a couple of days, but the numbers appear to be how much damage each single attack will do. (specifically, when you're in a Gym battle).
Your first Pidgey will do the most damage in combat. The final one will do the least.
The numbers represent the strength. The charging bars mean you have enough energy for your special attack (the second one, with the blue bars next to it.) Tap and hold while in battle to use the special attack.
The xp is 10% of the prestige that you get.
When training at a gym you will get 500 prestige per pokemon you defeat with equivalent CP value. ie. If you beat a 100cp pokemon with a 100cp pokemon, you will get 500 prestige.
You will get more than that if your Pokemon is lower than the one in the gym and less if it is higher.
How much less/more I am not exactly sure but as pointed out by Sobrique the formula may be P * D / A if the defending pokemon is of a higher CP. Where P is a multiplier of 500, D is the defender's CP and A is the attackers CP.
If the attacking Pokemon is higher then P is reduced to somewhere between 250 and 350.
The maximum you will get is 1000 prestige if your Pokemon is at least half the CP of the one in the gym. The minimum is 100 prestige which is when your Pokemon is double or more of the CP of the Gym's pokemon.
You cannot lose prestige by training at a friendly gym.
Best Answer
I only have personal experience to back this up, but it seems that the size of the special attack bar has little effect on the time it takes to charge a single segment. The main benefit of having an attack with several bars is that you can save up more special attacks to chain them together, doing a sort of rapid fire for special attacks. This can be useful if your special move is not super effective against the current opponent you're fighting, but will be against the next one in line once you beat the first.